Acoustic waveguide systems, wherein torsional, extensional, and/or flexural mode ultrasonic waves are launched by a transducer into a solid, rod-like, elongated elastic waveguide, are used to measure the density, level, and/or temperature of the fluid surrounding the waveguide. See U.S. Pat. Nos. 3,636,754; 3,514,747; 3,540,265; 4,452,334; and 5,456,114 incorporated herein by this reference. Depending on the waveguide cross section, the measured torsional wave transit time (echo) is a function of the density, viscosity, level, and temperature of the fluid surrounding the waveguide. The measured extensional wave transit time is primarily a function of the temperature of the waveguide.
In the prior art, however, the transducer usually consisted of electromagnetic coils surrounding a magnetostrictive segment of the waveguide. The coils are mounted about 10 centimeters apart along the magnetostrictive segment of the waveguide so that each coil can be biased and alternately driven to maximize the energy in each of the modes. The extensional and torsional mode acoustic waves are launched in the waveguide by means of the well-known Joule and Wiedemann effects, respectively. The Joule effect, or longitudinal magnetostriction, produces a longitudinal extension in the magnetostrictive rod when a pulse is applied to a coil surrounding the rod which produces a magnetic field parallel to the rod. The Wiedemann effect, or torsional magnetostriction, produces a torsional stress pulse in a magnetostrictive rod when a pulse is applied to a coil surrounding the rod which opposes a circumferential magnetic field surrounding the rod, as produced by a DC current flowing through the rod. Alternatively, since the magnetostrictive rod contains magnetic material, the rod may be initially conditioned to produce the circumferential magnetic field by polarizing the rod before assembly. A large DC current is passed through the magnetostrictive segment prior to its assembly to produce the proper polarization in the segment. Thus, the torsional wave may be launched directly using the coil. A permanent magnet adjacent the coil may be provided and aligned to cancel the effects of the circumferential field in the area of the coil. See also Ultrasonic Delay Lines, pp 135 and 140, ©1963 Brockelsby et al. Stable waveforms are desirable but for magneostrictive systems, when one secures the coil relative to the magnestrictor, spurious echoes are generated unless special precautions are taken.
Thus, prior art acoustic waveguide systems which produce torsional and/or extensional mode acoustic waves are fairly complex in construction and operation.